Several ideas on the origin of life have been put forward both scientific and non-scientific

For an idea to become a scientific theory it must be testable and a body of factual evidence must be accumulated in support of the theory.

The idea developed from a hypothesis that the conditions on the primitive Earth were not the same as those present today. Thus, we do not see spontaneous generation of life today because the necessary conditions no longer exist (A.I. Oparin and J.B.S. Haldane 1920s).

The combination of monomers, such as amino acids, into polymers, such as polypeptides, could have occurred when dry or highly concentrated monomers are heated. Condensation reactions take place forming

Clay mineral particles may have also contributed to the process.  Molecules adsorb to the clay particles (stick to the surface). The adsorbed molecules become concentrated together. Clay particles (coacervates) may have been essential catalysts in the formation of polymers.

Once formed polynucleotides show a tendency to copy themselves using complementary base pairing. This was probably catalysed by the presence of clay particles and metal ions. These single stranded polynucleotides would have been the equivalent of RNA.

• RNA was probably the first hereditary molecule having the ability to copy itself.

• RNA shows enzymic (catalytic) properties – called ribozymes. sRNPs that edit eukaryotic mRNA, cutting out the introns, are RNA based. The best-known ribozyme is the ribosome. The ribosome's active centre (where the amino acids are brought into place) is made entirely of RNA.

• Polynucleotides are very good molecules at storing and transmitting information but they lack the versatility for all the chemical functions of a cell.

• Polypeptides, which can form complex 3-dimensional structures (proteins), are much better at this.

• At some stage a partnership must have formed between the polynucleotides and the polypeptides where the polynucleotides directed the synthesis of the polypeptides.

• Today it is clear that information only flows from polynucleotides to polypeptides. Translation had started.

• Later the hereditary information was probably stored in the form of DNA which is more stable than RNA. The new partnership with proteins no doubt helped producing the necessary complex enzyme systems for transcription and replication.

• The passage of information from RNA to DNA is possible in nature. The reverse transcriptase enzyme of the retro viruses shows this.

If a piece of RNA codes for a particularly good protein (e.g. an enzyme to help replication) then there is nothing to stop that protein being used by other RNA molecules. So the advantage is lost. If however the RNA is enclosed in a membrane then it can keep it’s protein to itself and gains a selective advantage. So membranes probably pushed evolution by natural selection forwards.

• Membranes defined the first cell.

• The phospholipids are amphipathic molecules – one end is strongly hydrophobic the other is strongly hydrophilic. This means they form lipid bilayers when they are surrounded by water (rather than lipid droplets).

• All the components of a simple prokaryotic cell were now assembled. They diversified in their metabolism. By 2 billion years ago free oxygen was appearing in the atmosphere due to the activity of cyanobacteria and other photosynthetic bacteria. Chemosynthetic bacteria in what is today South Africa left gold deposits associated with organic carbon.

According to some scientists, evidence for prokaryotic cells is found as early as 3.9 billion years ago. The prokaryotes had the Earth to themselves for another 2.4 billion years, that is nearly 2/3rds of life’s history on Earth. They show an extraordinary diversity of biochemistry but structurally they are quite small and simple (1-10µm in diameter).

The amount of free oxygen in the atmosphere increased after the evolution of photosynthesis. It appeared from about 2 billion years ago and reached about 21% 1 billion years ago. Obligate anaerobes either became extinct or found niches where oxygen is absent.

A third possibility was open with the teaming up of microbes. Endosymbiosis – a large anaerobic cell teams up with an aerobic cell.  The aerobic prokaryote became a mitochondrion. Eukaryotic cells were formed, bigger and more complex, eventually forming multicellular organisms.

The evidence for endosymbiosis is strong

• Certain eukaryotic organelles have their own DNA that is a single naked loop of DNA, like the prokaryotes.

• But the amount of hereditary information is a lot less than free-living prokaryotes.

• These organelles have their own ribosomes that are smaller (70S) than those in the cytoplasm (80S). They are the same size as those in prokaryotes.

• The protein synthesis of these organelles is semi-independent of that taking place in the cytoplasm and it is inhibited by the same antibiotic that affects prokaryotes (chloramphenicol).

• These organelles are found in membrane envelopes as though they were captured in a vacuole or vesicle by a larger cell.

• These organelles are about the same size as a prokaryotic cell.

The idea is that mitochondria represent an aerobic prokaryote that took up residence in a larger cell. These are found in all the eukaryotic kingdoms (plants, animals, fungi and protoctista).

Chloroplasts represent a cyanobacterium type of prokaryote that was trapped in ancestral plants and some protoctista.

Some scientists think that bacteria called spirochetes are the ancestors of eukaryotic flagellae found in plant, animal and protoctista kingdoms. However, no trace of extra nuclear DNA has been found associated with flagellae.

1. Special creation – some supernatural being brought life into existence from nothing.

This can range from the literal interpretation of religious documents to "scientific" creationism.

Archbishop Ussher of Armagh in 17^th century used biblical references to support the idea that the Earth was created by God at 6:00pm on 22^nd October 4004BC.

Basing their ideas on those of William Paley (1743-1805) creationists have tried to imply that life is so complex and specialised that it could only have come about through the design of a supernatural creator (intelligent design). They accept that changes may occur within a species but reject the fact that new species can come about by the natural selection of chance mutations.

NB A scientific fact is an observation that has been repeatedly confirmed and for practical purposes is accepted as true.

As these ideas are developed from a theological view point (i.e. based upon belief) they are impossible to test. Most creationists arguments have been focussed on trying to disprove the theory of evolution by natural selection rather than by testing their own hypothesis.

Reference: Scientific American July 2002 Answers to creationist nonsense John Rennie. www.sciam.com

The support for this depends up on evidence that life exists elsewhere than on Earth and the evidence that it may travel through open space.

• Evidence for extraterrestrial life is weak so far.
• Mars Viking Probe (1976) revealed conflicting evidence of life and the Mariner Probe (1997) did not reveal any more evidence.
• Venus has inhospitable conditions (surface temperatures of over +400°C)
• The moons of Jupiter and Saturn could provide the right conditions, Europa appears to be covered in ice and Io shows volcanic activity.
• Bacteria, inadvertently left on a lunar probe, were collected by an Apollo mission and cultured successfully after nearly 2 years in space.
• Meteorites of Martian origin show that its early atmosphere would have been similar to Earth’s early atmosphere. They also showed (debatable) evidence of bacteria transported by meteorites.
• Life has been shown to exist on Earth in very inhospitable conditions that could exist on other planets (e.g. Antarctic dry valleys, mid-ocean ridges).
• Earliest evidence of life on Earth (3.9 billion years ago) is getting closer and closer to the origin of the Earth itself (4.5 billion years ago), which leaves little time for biochemical evolution.
• If it can be shown that life came from an extraterrestrial source the question still remains: How did life evolve in the first place?

This theory obtained powerful support after the development of the microscope and the observation that microbes could apparently appear from nowhere in a culture medium.

The theory was destroyed by the brilliant experimental work of Louis Pasteur who showed that microbial growth was due to contamination by spores from the atmosphere.